As shown in FIG. 23, a belt transmission device for transmitting the rotation of a crankshaft of an engine to various engine accessories such as an alternator includes a pulley arm 83 provided on the slack side of a belt 81 so as to be pivotable about a shaft 82, a tension pulley 84 rotatably supported on the pulley arm 83 at its end remote from the shaft 82, and a hydraulic auto-tensioner A configured to apply an adjusting force to the pulley arm 83, thereby biasing the pulley arm 83 in a direction in which the tension pulley 84 is pressed against the belt 81, whereby the tension of the belt 81 is kept constant.
A conventional hydraulic auto-tensioner A used in such a belt transmission device is disclosed in JP 2009-275757A and JP 2009-191863. In such a hydraulic auto-tensioner, a lower part of a rod is slidably inserted in a sleeve upstanding from a bottom surface of a bottomed cylinder accommodating a hydraulic oil to define a pressure chamber in the sleeve. A return spring is incorporated between a spring washer provided at an upper part of the rod and the bottom surface of the cylinder to bias the rod and the cylinder in the direction in which the rod protrudes from the cylinder.
Furthermore, a tubular spring cover that covers an upper part of the return spring is provided at a lower part of the spring washer, and a seal member is mounted in an upper part opening of the cylinder such that its inner periphery is in elastic contact with the outer periphery of the spring cover, thereby defining a sealed reservoir chamber between the cylinder and the sleeve. The reservoir chamber communicates with the pressure chamber through oil passages formed at a bottom portion of the cylinder. A check valve is mounted in a lower end opening of the sleeve so as to close when the pressure in the pressure chamber increases, thereby disconnecting communication between the pressure chamber and the oil passages.
This hydraulic auto-tensioner further includes a coupling piece provided at a lower surface of the cylinder and configured to be pivotally coupled to an engine block, and a coupling piece provided at an upper surface of the spring washer and configured to be coupled to the pulley arm 83 shown in FIG. 23, and is configured such that when a pushing force tending to push the rod into the cylinder is applied from the belt 81 through the tension pulley 84 and the pulley arm 83, the check valve closes, thus causing hydraulic oil sealed in the pressure chamber to flow through a leakage gap defined between the sliding surfaces of the sleeve and the rod to generate a hydraulic damper force in the pressure chamber by the viscous resistance of the hydraulic oil flowing through the leakage gap. The pushing force is thus damped by the hydraulic damper force.
In the hydraulic auto-tensioner described in JP 2009-191863A, the rod includes a valve fit-in hole opened at a lower end thereof and an oil path through which an upper part of the valve fit-in hole communicates with the reservoir chamber. A relief valve is mounted in the valve fit-in hole, and is configured such that a valve body of the relief valve is opened if the pressure in the pressure chamber becomes higher than a set pressure of the relief valve, thereby allowing hydraulic oil in the pressure chamber to flow through the oil path into the reservoir chamber.
In the hydraulic auto-tensioner according to JP 2009-191863A, the pressure in the pressure chamber can be held at a value not exceeding the set pressure of the relief valve, and hence the belt can be prevented from becoming over-tensioned.
In either of the hydraulic auto-tensioners described in JP 2009-275757A and JP 2009-191863A, the sleeve fit-in hole is formed at the bottom surface of the cylinder, and a lower end portion of the sleeve is press-fitted into the sleeve fit-in hole to hold the assembled state. Thus, the interference due to press-fitting is relatively large and the sleeve is press-fitted while rubbing against the sleeve fit-in hole when press-fitting the sleeve, whereby press-fitting burrs may form at the outer peripheral portion of the lower end face of the sleeve by such press-fitting.
In an hydraulic auto-tensioner, the lower end face of the sleeve is positioned at a higher level than the lower surface of the valve seat of the check valve incorporated in the lower end portion of the sleeve, and a gap is formed between the lower end face of the sleeve and the bottom surface of the sleeve fit-in hole. Thus, when burrs formed at the time of press-fitting drop, the burrs flow into the oil passages through the gap and mix into and float in the hydraulic oil. Burrs thus may get stuck in the leakage gap or the check valve, inhibiting the damper function when damping the pushing force applied on the hydraulic auto-tensioner.
An object of the present invention is to provide a hydraulic auto-tensioner for use with an engine accessory that prevents press-fitting burrs produced when fitting the sleeve from mixing into the hydraulic oil.